U.S. patent number 4,282,718 [Application Number 06/075,041] was granted by the patent office on 1981-08-11 for evaporator inlet water temperature control system.
This patent grant is currently assigned to Borg-Warner Corporation. Invention is credited to Richard A. Erth, Kenneth J. Kountz, Dean K. Norbeck.
United States Patent |
4,282,718 |
Kountz , et al. |
August 11, 1981 |
Evaporator inlet water temperature control system
Abstract
An evaporator inlet water temperature control system for
controlling the inlet water temperature of an evaporator utilized
in an inverter driven centrifugal chiller. The system creates the
proper chilled water set point at the evaporator outlet needed to
maintain the inlet water at its set point for greater efficiency of
operation. Control is accomplished by generating an electrical
error signal which is a function of an inlet water set point
voltage and the actual inlet water temperature. A logic signal is
produced as a function of the error signal detected by the circuit
and establishes a dead zone band representing a range in which the
error signal is acceptable. Output of the control system is passed
to the capacity control circuit of an inverter driven centrifugal
compressor chiller. When the inlet water temperature reaches its
set point during pull-down, the logic output of the circuit sets a
flip-flop which enables transmission gates to control changes in
the outlet water set point as needed to balance the load of the
cooling system.
Inventors: |
Kountz; Kenneth J. (Hoffman
Estates, IL), Erth; Richard A. (York, PA), Norbeck; Dean
K. (York, PA) |
Assignee: |
Borg-Warner Corporation
(Chicago, IL)
|
Family
ID: |
22123162 |
Appl.
No.: |
06/075,041 |
Filed: |
September 12, 1979 |
Current U.S.
Class: |
62/115; 62/209;
62/201; 62/228.1 |
Current CPC
Class: |
G05D
23/1931 (20130101); G05D 23/24 (20130101); F25B
49/022 (20130101); F25B 49/025 (20130101); Y02B
30/70 (20130101); Y02B 30/741 (20130101); F25B
2600/021 (20130101) |
Current International
Class: |
G05D
23/24 (20060101); G05D 23/20 (20060101); F25B
49/02 (20060101); F25B 005/00 (); F25D
005/00 () |
Field of
Search: |
;62/185,228R,228B,201,115,209 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wayner; William E.
Attorney, Agent or Firm: Hunter; Thomas B.
Claims
What is claimed is:
1. An evaporator inlet water temperature control system for a
refrigeration system including a compressor having an adjustable
capacity control means, a condenser and an evaporator having an
inlet receiving fluid from a load and an outlet discharging chilled
fluid to be returned to said load comprising:
means for sensing the temperature of said fluid at the evaporator
inlet,
means for sensing the temperature of said fluid at the evaporator
outlet,
a first control circuit means coupled to said outlet fluid
temperature sensing means to regulate said adjustable capacity
control means in response to the temperature of said outlet fluid,
said control circuit having a variable set point,
means for establishing a set point temperature for the fluid at the
evaporator inlet, and
a second control circuit means coupled to said inlet fluid
temperature sensing means and said set point temperature means to
generate a differential electrical signal as a function of the
difference between the sensed temperature and the set point
temperature of the fluid,
said second control circuit means having an output coupled to said
first control circuit and dependent on said electrical signal for
adjusting the variable setpoint and regulate the adjustable
capacity control means to maintain the inlet set point
temperature.
2. The system according to claim 1 wherein said output of said
second control circuit regulates the adjustable capacity control
means to maintain the level of inlet set point temperature within a
dead-band range of acceptable error.
3. The system according to claim 1 wherein the output of the second
control circuit means establishes a fluid temperature at the outlet
of the evaporator sufficient to maintain said inlet set point
temperature.
4. The system according to claim 3 wherein the output of said
second control circuit means maintains a predetermined fluid
temperature differential between the inlet and outlet of the
evaporator during the pull-down phase.
5. The system according to claim 1 wherein said second control
circuit means includes means to create electrical signals
representative of said inlet temperature and said set point
temperature.
6. The system according to claim 5 wherein said second control
circuit means including means to combine said electrical signals to
generate said differential electrical signal.
7. The system according to claim 1 wherein said second control
circuit means includes a logic circuit responsive to said
differential electrical signal to control said output.
8. The system according to claim 7 wherein said logic circuit
includes a flip-flop device, said flip-flop device being activated
after pull-down of the refrigeration system from initial activation
when the inlet temperature has reached set point.
9. The system according to claim 7 wherein said second control
circuit means include an output circuit controlled by said logic
circuit, said output circuit producing said output to establish an
initial evaporator outlet temperature at a predetermined value
lower than the inlet set point temperature.
10. The system according to claim 8 wherein said logic circuit
includes pulse producing means to create a periodic data pulse to
periodically modify control of said output circuit.
11. The system according to claim 10 further including a pair of
amplifier means having an input coupled to said differential
signal, said amplifier means having different voltages applied to
the respective inputs to form a dead zone logic signal representing
acceptable error between actual and set point inlet
temperature.
12. The system according to claim 9 wherein said output circuit
includes transmission gate means controlled by said logic
circuit.
13. The system according to claim 12 wherein said logic circuit
periodically releases said transmission gate means to modify said
output in response to said electric differential signal.
14. The system according to claim 1 further including pulse
generating means coupled to gate means, said gate means having an
input coupled to said differential electric signal, and said pulse
generating means generating pulses to periodically release said
gate means and produce a pulse output having height dependent on
said differential electric signal.
15. The system according to claim 14 wherein said pulse output is
applied to the input of integrating circuit means, the output of
said integrating circuit controlling said output.
16. A method of maintaining the evaporator inlet temperature of a
centrifugal compressor operated refrigeration system at a set point
comprising the steps of
deriving a signal which represents the difference between actual
inlet temperature and set point temperature,
creating a logic signal for the temperature error representing said
difference,
establishing an initial outlet temperature of said evaporator
during pull-down of the refrigeration system after activation from
a rest state, and
applying said logic signal to temperature control means to modify
said evaporator outlet temperature to maintain inlet set point
temperature after the inlet set point temperature is attained after
pull-down.
Description
BACKGROUND OF THE INVENTION
A. Field of the Invention
This invention relates in general to cooling systems and, in
particular, to a capacity control system for regulating
chillers.
More specifically, without restriction to the particular use which
is shown and described, this invention relates to an evaporator
inlet water temperature control system for inverter-driven
centrifugal compressor chillers by which evaporator inlet water
temperature is controlled by creating the proper evaporator outlet
chilled water temperature.
Large capacity air-conditioning systems commonly have employed
centrifugal compressors having guide vanes which are adjustable to
control the capacity of the compressor for effective and efficient
operation. Control of compressor capacity through adjustment of the
vanes alone, has proved to be unsatisfactory because of its
resultant low efficiency operation of the system. In an effort to
overcome such problems, attempts have been made to control not only
the position of the pre-rotation vanes, but to regulate the speed
of the electric motor driving the compressor for purpose of an
energy effective system and to avoid surge.
One highly effective system which relies on vane adjustment and
speed control to achieve superior capacity control of a centrifugal
compressor chiller, is disclosed in U.S. Pat. No. 4,151,725 to
Kountz et al for A Control System for Regulating Large Capacity
Rotating Machinery and assigned to the assignee of this invention.
In the system disclosed in U.S. Pat. No. 4,151,725, a control
system effectively maximizes efficiency without encountering surge
problems by monitoring the temperature of the refrigerant in the
condenser discharge line, the temperature of the saturated
refrigerant leaving the evaporator, the temperature of the chilled
water discharged from the evaporator of the chiller, and the inlet
guide vane position. Based on the foregoing four parameters and a
set point temperature input, the control system described in U.S.
Pat. No. 4,151,725 effectively regulates the refrigeration system
by regulating the speed of the compressor and adjusting vane
position.
The temperature of the chilled water sensed at the outlet from the
evaporator in U.S. Pat. No. 4,151,725 is applied to a circuit which
also receives a temperature set point signal from a potentiometer
or another suitable unit, such as a thermostat in the building
space. Thus, a signal is derived representing the difference, if
any, between the set point temperature of the thermostat and the
like and the instantaneous load. Greater efficiency could be
attained in such a refrigeration system, however, if the inlet
water being directed to the evaporator on return from the building
is constantly monitored and the capacity control system is
responsive thereto to maintain a constant or set point temperature
at the inlet regardless of the load conditions. Maintenance of the
set point inlet temperature of the water to the evaporator in
response to actual load conditions would result in significantly
improved efficiency and economy of operation since a variable input
related directly to the temperature of the water after circulation
is applied to the capacity control system.
B. Prior Art Patents
U.S. Pat. No. 4,112,703 to Kountz discloses a refrigeration control
system in which a controlled space is maintained at a set point by
adjusting the refrigerant temperature at the evaporator outlet in a
refrigeration system. In U.S. Pat. No. 3,751,940 to Norbeck, a
refrigeration system is disclosed having circuit means for
automatically energizing the PRV motor.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to maintain the inlet
water temperature to a compressor of a chiller at a predetermined
set point.
Another object of this invention is to maintain the inlet water to
an evaporator of a chiller at a set point by regulating the water
temperature at the evaporator outlet.
A further object of this invention is to achieve pull-down of a
chiller to an inlet set point temperature of an evaporator by
establishing a predetermined temperature difference between the
inlet set point and the initial outlet set point of the
evaporator.
Still another object of this invention is to generate an adaptive
set point to maintain exit evaporator temperature as a differential
function of inlet water set point temperature and actual water
input temperature.
These and other objects are attained in accordance with the present
invention wherein there is provided an evaporator inlet water
temperature control system for an inverter driven centrifugal
compressor. The set point temperature at the inlet to the
evaporator is maintained by the system of the invention by creating
the proper temperature of the chilled water at the evaporator
outlet. The system herein disclosed includes means to develop a
differential signal as a function of actual inlet temperature and
inlet set point temperature which is compared to first and second
reference voltages to establish a dead zone band in which an error
signal is acceptable. During pull-down of the system until the
evaporator inlet temperature reaches set point, the inlet and
outlet temperature of the evaporator are maintained at a
predetermined temperature difference. Upon the inlet temperature
reaching the set point after pull-down, flip-flop circuit means are
provided to cause outlet temperature to be regulated to maintain
inlet temperature within a dead zone band of acceptable error
during load conditions.
DESCRIPTION OF THE DRAWINGS
Further objects of the invention, together with additional features
contributing thereto and advantages accruing therefrom, will be
apparent from the following description of several embodiments of
the invention which are shown in the accompanying drawings with
like reference numerals indicating corresponding parts throughout,
wherein:
FIG. 1 is a block diagram illustrating the incorporation of the
evaporator inlet water temperature control system of the invention
in a capacity control system to regulate the operation of a cooling
system;
FIG. 2 is a schematic diagram which illustrates the circuit details
of the evaporator inlet water temperature control system of FIG.
1;
FIG. 3 is a graphical illustration useful in understanding the
operation of the invention shown in FIG. 2; and
FIG. 4 is a schematic circuit diagram of another embodiment of the
evaporator inlet water temperature control system of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
Although not intended to be so limited, the evaporator inlet water
temperature control system of the invention is shown in FIG. 1 as
being incorporated in the control system for regulating large
capacity rotating machinery as disclosed in U.S. Pat. No.
4,151,725, to which reference is specifically made herein. In FIG.
1 there is illustrated the control system and cooling equipment in
block diagram shown and described in U.S. Pat. No. 4,151,725
employing the evaporator inlet water temperature control system of
the invention. In the following description, a general description
is set forth of the components and related function of the cooling
system shown in FIG. 1 to which the inlet temperature control
system herein disclosed may, for example, supply a control signal
to achieve an evaporator outlet temperature at a level to maintain
a set point inlet temperature.
FIG. 1 depicts certain conventional components of a cooling system,
such as a centrifugal compressor 20 for passing refrigerant (such
as R-11 or another suitable medium) through line 21 to a condenser
22. In the condenser the water from the cooling tower passes from
line 23 into the condenser, and is returned over line 24 to the
cooling tower, or to the other head rejection means. The
refrigerant at the discharge side of the condenser 22 is passed
over line 25, through a fixed orifice 26, and line 27 to the
refrigerant inlet connection of the evaporator. The refrigerant
passes through the evaporator and out the duct 30, which includes a
plurality of inlet guide vanes 31 positioned as shown. In this
description the inlet guide vanes are termed PRV, or prerotation
vanes, and the position of the PRV is regulated by a small motor 32
which receives a control signal over a plurality of conductors,
here represented as a single line 33. Those skilled in the art will
readily appreciate that a plurality of conductors are represented
by the single lines of FIG. 1. The higher temperature water from
the building (or other cooling load) is returned over line 34,
cooled in the evaporator 28, and the chilled water is returned to
the building over line 35.
An induction motor 36 is coupled over shaft 39 to the centrifugal
compressor 20, and this motor 36 is itself driven from an inverter
37. The inverter receives a d-c input voltage over line 38, thus
determining the amplitude of the inverter output voltage. A voltage
control circuit 40 is provided between a voltage supply line 41 and
line 38 which passes the voltage to the inverter. This can be a
conventional circuit, such as a phase-controlled rectifier circuit,
which receives an input a-c voltage on line 41 and provides a d-c
voltage on line 38 which is regulated in accordance with the signal
received over line 42. If no regulation is necessary, a d-c voltage
can be supplied over line 38 to the inverter from batteries, a
transformer-rectifier, or any suitable source. The frequency of the
inverter output voltage is regulated by the periodicity of the
timing signals, or gating signals, supplied over line 43 from a
logic circuit 44. This is a well-known circuit which receives a
regulating signal on line 45 and utilizes this regulating signal to
govern the frequency of the pulses supplied on line 43. One
well-recognized arrangement receives a d-c voltage as a control
signal on line 45, and a voltage-controlled oscillator in the logic
circuit 44 provides pulses at a frequency determined by the
amplitude of the signal on line 45. The logic circuit generally
includes a ring-counter type circuit to distribute the pulses to as
many thyristors or other switches as are used in the inverter
circuit.
The control system 50 is utilized to regulate not only the speed of
induction motor 36, but also the physical position of the
pre-rotation vanes 31, by a speed control signal supplied over line
51 and a vane position (drive open or drive closed) signal supplied
over line 33. The circuit arrangement of the invention insures that
surge is avoided, and that the compressor is regulated in the most
energy-efficient manner. In this embodiment the speed control
signal is a d-c voltage supplied from an integrating circuit 52,
and the vane control signal can be either an "open vanes" signal on
line 53 of a "close vanes" signal on line 54, or no signal ("hold
vanes"). These output control signals are derived from different
input signals, including a first signal on line 55 which is
provided by a thermistor or other temperature sensing unit 56
positioned to contact the refrigerant in the condenser discharge
line as shown. A second signal is provided on line 57, obtained
from a second-sensing means or thermistor 58 which is exposed to
the saturated refrigerant vapor leaving the evaporator. The first
and second signals are combined in a summation means 59, which can
be a differential amplifier circuit, to provide a resultant signal
on line 60 which connotes the head of the compressor. A suitable
means in the control system which utilizes this signal, and the
other input signals, is described in detail in U.S. Pat. No.
4,151,725.
A potentiometer 61 is shown with its movable arm or wiper
mechanically coupled to the PRV, or to the output shaft of motor 32
which drives the PRV. Thus, the electrical signal on line 62
indicates the physical position (fully open, 3/4 open, and so
forth) of the inlet guide vanes in a continuous manner. The inlet
guide vane position signal can be appropriately combined with the
compressor-head-indicating signal to assist in regulating the
compressor operation.
A third temperature sensing means, which can be another thermistor
63, is positioned to sense the temperature of the chilled water
discharged from the evaporator 28. Thermistor 63 thus provides a
third signal, which is applied over line 64 to another differential
amplifier stage 65, which also receives an evaporator inlet
temperature set point signal from a fourth temperature sensing
means such as, for example, in the form of a thermistor 66.
Thermistor 66 is positioned in line 34 at the inlet of evaporator
28 to sense the temperature of the water on its return from its
circuit through the building or other environment in which the
system is operating.
The inlet temperature sensed by thermistor 66 is directed to the
evaporator inlet water temperature control system of the invention
which is generally designated by reference numeral 80 and will be
described in detail later. Inlet temperature control circuit 80
monitors the evaporator inlet temperature in comparison to a
selected inlet set point temperature and provides control system 80
with a signal to establish the proper outlet temperature of the
evaporator to maintain the inlet water at its set point. Thus, the
output signal on line 67 represents the difference, if any, between
the condition called for (denoted by the signal derived from inlet
control circuit 80) and the instantaneous load condition
(represented by the signal on line 64).
In the foregoing description, a general perspective of a known
refrigeration arrangement and the control system has been provided.
For more specific details as to the circuitry and function of
control system 50 utilized in controlling operation of the cooling
system through adjustments of the inlet guide position of the PRV
and speed of motor 36 of compressor 20, reference is made
specifically to U.S. Pat. No. 4,151,725.
Referring now to FIG. 2, there is illustrated circuit details in
schematic form of one embodiment of the evaporator inlet water
temperature control system 80 of the invention to establish an
outlet water temperature of the evaporator at a level sufficient to
maintain the set point of the water inlet temperature of the
evaporator. In FIG. 2, the inlet water temperature to the
evaporator sensed by thermistor 66 is passed over line 81 and one
of the 24 k resistors to an input terminal of a differential
amplifier 82. The signal from the set point potentiometer 83,
representing the set point inlet water temperature, is passed over
line 84 and another 24 k resistor to the other input connection of
stage 82 to sum the error between the set point inlet water
temperature established by potentiometer 83 and the actual inlet
temperature sensed by thermistor 66. A nominal d-c voltage of 5.50
volts is applied to the input of differential amplifier 82 and the
output of amplifier 82 is 5.50 volts when there is no temperature
error. When the output is other than 5.50 volts, this represents
the temperature error signal on line 85 equal to the difference
between set point and actual temperature. The signal on line 85 is
passed to amplifier stages 86 and 87 to provide logic signals on
lines 88 and 89 when the difference between the inlet set point
temperature and inlet water temperature exceeds the amount of the
dead band, or the dead zone area. In the illustrated embodiment,
the dead zone was set to include a temperature difference of
.+-.0.1.degree. F., electrically represented by the difference
between 5.12 volts and 5.86 volts applied to the stages 86 and 87.
The expected voltage versus temperature relationship for the inlet
thermistor 66 is: ##EQU1##
The signal from stage 86, representing a dead zone logic signal A*,
is passed over line 88 to an inverter 90, and as an input to a NAND
gate 91. The inverted logic signal A*, after passing through
inverter 90, is directed to a flip-flop 92 which is set in a manner
to be described. The output signal from flip-flop 92 in the form of
logic A*.sub.mem is passed over line 93 to an inverter 94 and
subsequently by line 95 as one input to a NOR gate 96. The other
dead zone logic signal B* formed by stage 87 is passed over a line
89 to one input of a NAND gate 100. The logic signal formed by
flip-flop 92 on line 93 is coupled to second inputs to NAND gates
91 and 100. The third input of NAND gates 91 and 100 is provided by
a timer chip circuit 101 which passes a logic signal F* to NAND
gates 91 and 100 over line 102. Logic F* may be a data pulse of
width of 1-2 seconds and having a period of 2-5 minutes, the
function of which will be described later. The outputs from NAND
gates 91 and 100 are respectively directed through inverters 103
and 104 and form respective logics. The signal created by NAND gate
91 and inverter 103 represents a logic Ld whereby
Ld=F*A*A*.sub.mem. NAND gate 100 and inverter 104 create logic Li
whereby Li=F*B*A*.sub.mem. Ld is a logic signal to decrease
evaporator outlet temperature set point and Li is a logic signal to
increase outlet water temperature set point. In addition, the
logic, Lm, to maintain the outlet set point temperature is defined
by
where Lr=A*.sub.mem.
Logic Ld is passed over line 105 as a second input to NOR gate 96
and as input to a transmission gate 106 having approximately 4.5
volts applied thereto. The logic Li is passed over line 107 as the
third input to NOR gate 96 and further controls a second
transmission gate 108 and release of approximately 6.5 volts. A
third transmission gate 109 controlled by the logic signal Lm from
NOR gate 96, controls release of a voltage of 5.5 volts. It should
be apparent to one skilled in the art that release of gates 106,
108 and 109 is determined by logic signals, Li, Ld and Lm,
respectively.
The output of potentiometer 83, representing the evaporator inlet
set temperature, is passed over line 110 to the positive input of
amplifier 111 having a voltage of 4.695 volts applied to its
negative input. Another amplifier stage 112, having a 5.5 d-c
voltage being applied to its positive input, directs a 5.5 volt
nominal signal, during pull-down of the system, over line 113 to be
combined with the output from amplifier 111 passed over line 114
whereby the initial value of the outlet water set point voltage is
0.805 volts higher (i.e., 10.degree. F. lower) than the set point
voltage of the inlet water. The amplifier stage 112 includes a
transmission gate 115 connected thereacross and released by logic
A*.sub.mem during pull-down to produce the 5.5 volts nominal
output. After pull-down of the system, amplifier 112 then acts as
an integrating circuit as will be apparent. The combined outputs
from stages 111 and 112 are passed through amplifier stage 120
which produces a signal representing outlet water temperature set
point to be passed by line 121 as an input to differential
amplifier 65 of capacity control circuit 50 of FIG. 1. Amplifier 65
in control circuit 50 corresponds to amplifier 65 as shown in FIG.
6A in the above-identified Kountz et al. U.S. Pat. No. 4,151,725.
In effect, the fixed 24 K resistor coupled to terminal 10 of
amplifier 65 in Kountz et al. becomes, in the present invention, a
variable resistance controlled by the evaporator inlet water
temperature control circuit.
As shown in FIG. 3, the higher voltage of additional 0.805 volts as
the initial outlet set point temperature establishes from initial
start-up the pull-down function of outlet and inlet temperature
which continues until such time as the set point of the inlet water
is attained. During the pull-down phase, logic signal A* is logical
high (1) whereby the logics applied to transmission gates 106, 108
and 109 are all logical low (0). During pull-down, it should
further be apparent to one skilled in the art that the logic signal
passed over line 95 from inverter 94 is high and releases
transmission gate 115 connected across amplifier 112.
When the inlet water set point has been reached after pull-down as
graphically represented in FIG. 3, logic signal A* goes low and
thereby sets flip-flop 92 forming the logic signal A*.sub.mem. When
logic signal A*.sub.mem goes high the logic on line 95 is low to
open gate 115 across amplifier 112. Thereafter, gates 106 and 108
are selectively released to alter outlet water set point to
maintain the wave form shown in FIG. 3 after an initial increase in
the outlet temperature after set point has been passed. After
pull-down, the logic A* goes low and A* goes high as shown in FIG.
3 while the .DELTA.T required by the load is established.
Thereafter, the formation of logic signals Lm, Ld and Li operate to
control gates 106, 108, and 109 as required to balance the load and
maintain evaporator inlet temperature. The logic F* from the timer
chip circuit 101 during a pulse forms an input to NAND gates 91 and
100 and controls whether one of the signals Ld or Li are logical
high. Between pulses of logic F*, three inputs of logic low are
applied to NOR gate 96 to release gate 109. Thus, the alternate
voltages of pulsed nature are applied to integrator 112. The output
from integrator 112 is passed on line 113 and is combined with the
signal from amplifier 114 to maintain inlet evaporator
temperature.
Referring now to FIG. 4, there is illustrated still another
embodiment of the evaporated inlet water temperature control system
of the invention, generally designated by reference numeral 150.
The control system 150 of FIG. 4 performs a similar function to
regulate the inlet water temperature as was described with
reference to the control system shown in FIGS. 2 and 3. The output
of the control system 150 is coupled to the capacity control system
50 as described in connection with the preceding embodiment of the
invention. The temperature of the inlet water to the evaporator 28
shown in FIG. 1 is sensed by thermistor 66a and compared to an
inlet temperature set point represented by potentiometer 83a and a
difference between the actual temperature and the set point
temperature is formed by differential amplifier 82a as in the
previous described embodiment. The output from differential
amplifier 82a is passed over line 84a to a gate 151 and through a
comparator circuit 152 which provides a logic signal A* on line 153
that is logical zero when the difference between inlet temperature
is greater than the acceptable set selected zone determined by the
5.124 volts applied to stage 152. The logic signal on line 153 is
directed to a flip-flop 154 similar in function to flip-flop 92
previously described in FIG. 2. Flip-flop 154 passes on lines 155
and 156 respectively, a logic signal A*.sub.mem. The logic signal
passed over line 156 is coupled to a pulser 160 which generates a
pulse output similar to the pulse output of timing chip circuit 101
of the preceding embodiment. The other logic signal formed by
flip-flop 154 on line 155 is passed to a transmission gate 161
connected across amplifier 112a. The function of amplifier 112a is
identical to amplifier 112 described with reference to FIG. 2. The
output from amplifier 112a is passed over line 113a to amplifier
stage 123a of similar design as amplifier 123 in the previous
embodiment.
Similarly as described with reference to the embodiment of FIG. 2,
a signal is passed over line 110a through an amplifier 111a which
acts to subtract a voltage of 4.695 from the 5.5 set point voltage
of the inlet water temperature. During pull-down, the control
circuit of FIG. 4 operates to permit the temperature of inlet water
to reach set point as determined by the voltage supplied by
amplifiers 112a and 111a to stage 123a, which is 0.805 volts higher
than the set point inlet temperature. When the inlet temperature
reaches the set point reference voltage, the signal on line 153
goes logical high whereby the flip-flop circuit causes the signal
on line 156 to go logical high to generate pulses from pulser 160
while logic on line 155 goes low to close transmission gate 161.
Thus, the pulser 160 generates an output signal to control release
of transmission gate 151 and apply a pulsed signal to integrator
112a. The pulse height is dependent upon the error magnitude coming
from differential amplifier 82a representing the difference between
actual inlet temperature and set point inlet temperature. The rate
of change in the integrator output directed to amplifier 123a over
line 121a is dependent upon the magnitude of temperature error, to
provide an input to control system as previously noted. The pulses
from pulser 160 may be terminated directly by resetting pulser 160
and flip-flop 154.
While the invention has been described with reference to preferred
embodiments, it will be understood by those skilled in the art that
various changes may be made and equivalents may be substituted for
elements thereof without departing from the scope of the invention.
In addition, many modifications may be made to adapt a particular
situation or material to the teachings of the invention without
departing from the essential scope thereof. Therefore, it is
intended that the invention not be limited to the particular
embodiments disclosed as the best mode contemplated for carrying
out this invention, but that the invention will include embodiments
falling within the scope of the appended claims.
* * * * *